1. Field of the Invention
This invention relates to the polarization-dependent, hollow-core, optical fibers and, more particularly, to the control of transverse optical modes by their polarization state in such fibers.
2. Discussion of the Related Art
A transverse optical mode propagating in the core of an optical fiber is typically composed of two orthogonal polarization states. If the mode is birefringent, the orthogonal polarization states propagate with different effective index values (or propagation constants). On the other hand, if the mode is not birefringent, the two polarization states propagate with the same effective index value (propagation constant).
Control of these polarization states (or simply polarization) is important, for example, in the design of optical amplifiers. In addition, polarization dependence may be exploited in a variety of other optical devices such as polarization splitters, couplers, filters and the like. Thus, for example, polarization plays a significant role in the use of bandgap fibers, and in particular hollow-core fibers. Hollow-core fibers may be advantageous for communications applications since they have nonlinearities much lower than traditional solid-core fibers. However, hollow-core fibers typically have large accidental birefringence, which leads to differential delay between orthogonal polarizations. This delay may be unacceptable in some communications systems. Since accidental birefringence may not be eliminated easily, a system with intentional management of polarization may be preferred and may include polarization-maintaining fibers, polarizing fibers, and polarization-sensitive couplers or splitters. Hollow-core fibers may also be used in sensors, as dispersion elements in a pulse amplification system, etc. It is often desirable to control polarization in these applications as well. Polarization-maintaining, hollow-core fibers have already been incorporated in some experimental demonstrations, and polarizing or polarization-splitting fibers would also be useful.
Prior art designs exist for polarization-maintaining (birefringent) bandgap fibers and fibers with limited polarization dependent loss. These approaches have severe limitations in performance and functionality. For example, high birefringence or polarization-dependent loss are often accompanied by undesirable properties such as high signal loss, high surface interaction, limited bandwidth, non-circular mode shape, etc.
Thus, there is a need in the art for hollow-core optical fibers with polarization-controlling properties that alleviate or overcome one or more of the limitations of the aforementioned prior art. In particular, there is a need for optical fibers that are capable of controlling polarization-dependent loss and/or polarization-dependent coupling.